High-solids anaerobic membrane bioreactors as process-intensified platforms for circular organic-stream valorization: From membrane-scale processes to system-scale sustainability

High-solids anaerobic membrane bioreactors (AnMBRs) are receiving growing interest for the treatment of urban organic feedstocks, including sewage sludge, food waste, distillery stillage and dairy residues, achieving superior COD removal, stable operation, and enhanced methane yields. Demonstrated applications, including several full-scale implementations, show high treatment capacities and robust biogas production. Performance improvements have been explored through emerging process intensification strategies, including optimized operation, energy-lean pretreatments, biochar-based media, and electrochemically assisted configurations, with reported biogas yield improvements of 10–60%, although most remain at low technology readiness levels. At elevated solids concentrations, typically with mixed liquor total solids of 25–70 g/L and fluxes mostly below 10 L/m 2 /h, membrane fouling in AnMBRs is predominantly governed by the formation of compressible cake/gel layers, reduced shear efficiency at high viscosity, and mineral-associated scaling, rather than simple pore blocking, underscoring the importance of appropriate module configuration, material selection, and sustainable operating conditions that balance flux, shear, and cleaning. Moreover, coupling with post-treatment technologies, such as bipolar membrane electrodialysis and hydroxyapatite-enhanced partial nitritation/anammox, can shift AnMBRs from waste treatment systems to circular biorefineries. Environmentally, AnMBRs can contribute to reduced methane emissions, nutrient recovery, and water reclamation as part of integrated anaerobic treatment systems, whereas their economic viability remains constrained by high operational costs and substantial investments in membranes and advanced automation. Future research should integrate long-term operational data on membrane performance with techno-economic analysis and life-cycle assessment to assess scale-up stability and cost–impact trade-offs.